Transistor for active matrix display and a method for producing said transistor

ABSTRACT

A transistor for active matrix display and a method for producing the transistor ( 1 ). The transistor ( 1 ) includes a microcrystalline silicon film ( 5 ) and an insulator ( 3 ). The crystalline fraction of the microcrystalline silicon film ( 5 ) is above 80%. According to the invention, the transistor ( 1 ) includes a plasma treated interface ( 4 ) located between the insulator ( 3 ) and the microcrystalline silicon film ( 5 ) so that the transistor ( 1 ) has a linear mobility equal or superior to 1.5 cm2V−1s−1, shows threshold voltage stability and wherein the microcrystalline silicon film ( 5 ) includes grains ( 6 ) whose size ranges between 10 nm and 400 nm. The invention concerns as well a display unit having a line-column matrix of pixels that are actively addressed, each pixel comprising at least a transistor as described above.

This application is a Nation Stage Application of InternationalApplication PCT/EP2004/001107, filed Feb. 6, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a transistor for active matrix display, adisplay unit comprising the said transistor and a method for producingthe said transistor.

2. Description of Related Art

Since the advent of the portables and the need for flat display panels,electronic displays implementing thin-film transistor technology andliquid crystals have experienced a phenomenal growth, to the point wherefull-colour displays have been realised that can compete with cathoderay tube displays. Amorphous silicon thin-film transistors areextensively used as pixel charging devices in active matrix liquidcrystal displays, principally because of its application to large glasssubstrates, low cost and remarkable matching with the requirements ofliquid crystal driving. Over the last decade, a rapidly growing demandfor high information content displays offering high performances(excellent contrast, homogeneity of colours, high luminance, largeviewing angles . . . ) and having sizes down to “micro-panel”, formobile phones for example, has nonetheless raised a huge interest fornew technologies like organic light emissive diodes (OLED), polymermaterial based light emissive diodes (PLED), . . . The response time ofOLED devices makes them perfectly suitable for video rate.

This demand adds constraints on the active material used in thin filmtransistors for active matrix displays, namely a higher stability and amore rapid charging than amorphous silicon (a-Si:H) thin films canprovide. It is also necessary for a higher integration and to furtherreduce the cost of the display to process the driving circuits directlyon the glass panel instead of connecting external circuits.

It is known that microcrystalline silicon (μc-Si:H) is compatible withamorphous silicon technology and can be directly deposited using plasmadeposition technology at low temperatures without further thermal orlaser treatment.

However, up to date studies of μc-Si:H thin films {ROCA I CABARROCAS, Pet al.; J. Appl. Phys. 86 (1999) 7079 and references cited therein} haveonly reported linear mobilities similar to those of a-Si:H thin filmtransistors. Therefore, no improvement would be expected from thesestudies on the charging time of pixels using said μc-Si:H thin filmtransistors and on the driving circuit integration.

SUMMARY OF THE INVENTION

The purpose of the invention is hence to remedy the shortcomingsmentioned above and to propose a transistor for active matrix displayhaving one or more of the following features and advantages: namely, ahigh field effect mobility, an excellent threshold voltage stability, ahigh level of drive circuit integration and a high duty ratio, offeringa low cost transistor for pixel-charging devices used in active matrixdisplays.

In addition, the invention has as an objective a method for producing atransistor for active matrix display, this method being at once rapidand easy to implement, in particular, in industrial transistormanufacturing devices.

To this end, the invention concerns a transistor for active matrixdisplay comprising a microcrystalline silicon film and an insulator, thecrystalline fraction being above 80%.

According to the invention, said transistor comprises a plasma treatedinterface located between the insulator and the microcrystalline siliconfilm so that the said transistor has a linear mobility equal or superiorto 1.5 cm²V⁻¹s⁻¹ and shows threshold voltage stability.

The microcrystalline silicon film is composed of a mixture of amorphoustissue and crystallites that are crystallised grains. We shall callhereinafter “Crystalline fraction”, the ratio by volume of the saidgrains. At a crystalline fraction of 100%, microcrystalline siliconfilms without any amorphous phase are achieved. In other words, the thinfilm is fully crystallised.

We shall call hereinafter “Threshold voltage stability”, a thresholdvoltage shift equal or inferior to 0.5 V with time when the thin filmtransistor Is submitted to a bias stress. Typical stress tests areperformed for example under a gate voltage of 30 V and at a substratetemperature of 60° C.

According to various embodiments, the present invention also concernsthe characteristics below, considered individually or in all theirtechnical possible combinations.

-   -   the microcrystalline silicon film comprises grains whose size        ranges between 10 nm and 400 nm,    -   said grain size ranges between 100 nm and 200 nm,    -   the microcrystalline silicon film thickness is comprised between        100 nm and 450 nm,    -   said transistor has a top-gate electrode,    -   said transistor has a bottom-gate electrode,    -   the microcrystalline silicon film is produced by hot wire        technique,    -   the microcrystalline silicon film is produced by radiofrequency        glow discharge technique.        The invention concerns as well a display unit having a        line-column matrix of pixels that are actively addressed.        According to the invention, each pixel comprises at least a        transistor as previously described.

According to various embodiments, the present invention also concernsthe characteristics below, considered individually or in all theirtechnical possible combinations.

-   -   said pixels comprise light emissive organic materials,    -   said pixels comprise liquid crystals,    -   said pixels comprise light emissive polymer materials,    -   electronic control means to drive each pixel are at least        partially integrated on the corresponding microcrystalline        silicon film.

The display unit described above can be advantageously applied with adevice selected from the group comprising a computer, a video camera, adigital camera, a portable terminal, a player for recorder media, anelectronic game equipment and a projector. The invention concerns aswell a method for producing a transistor for active matrix displaycomprising the steps of forming an active material and electrodes, saidactive material being formed using vapor deposition methods and saidtransistor comprising an insulator. According to the invention,

-   -   one forms a plasma treated interface on top of said insulator,        and    -   one forms a microcrystalline film on top of said treated        interface at a temperature comprised between 100 and 400° C.        using at least a deposition chemical element and a        crystallisation chemical element.

According to various embodiments, the present Invention also concernsthe characteristics below, considered individually or in all theirtechnical possible combinations.

-   -   said plasma treated interface is selected from the group        consisting of a SiN_(x) layer, a SiN_(x)O_(y) layer, a SiO₂        layer and glass,    -   one forms the plasma treated interface using a gas selected from        the group consisting of N₂, O₂, N₂O and NH₃,        The insulator is treated by plasma deposition to form a plasma        treated interface so as to reduce the density of nucleation        sites.    -   the microcrystalline silicon film is formed using a buffer gas        selected from the group consisting of Ar, Xe, Kr and He,    -   said crystallisation chemical elements is H₂,    -   said deposition chemical elements are selected among the group        comprising SiH₄, SiF₄,    -   said deposition chemical elements flux and said crystallisation        chemical elements flux are at equilibrium during the growth of        the microcrystalline silicon film,    -   one forms a top gate transistor,    -   one patterns a substrate comprising a metallic layer to form        source and drain electrodes,    -   one forms a bottom gate transistor,    -   a substrate comprises a gate electrode,    -   the microcrystalline silicon film comprises grains whose size        ranges between 10 nm and 400 nm,    -   the microcrystalline silicon film thickness is comprised between        100 nm and 450 nm,    -   the vapor deposition methods use radiofrequency glow discharge        technique,    -   one uses a 13.56 MHz PECVD reactor.        A “13.56 MHz PECVD reactor” means here a reactor powered by        radiofrequency energy at a frequency of 13.56 MHz used with a        plasma enhanced chemical vapour deposition method.

BRIEF DESCRIPTION OF THE DRAWINGS

To facilitate further description of the invention, the followingdrawings are provided in which:

FIG. 1 is a schematic view of a thin film transistor structure for abottom gate transistor according to the invention.

FIG. 2 shows the experimental values obtained as a function of thepercolation thickness (nm) for the linear mobility of a μc-Si:H thinfilm produced from SiF₄-Ar-H₂ mixtures.

FIG. 3 shows an atomic force microscopy relief of a μc-Si:H thin filmproduced from SiF₄-Ar-H₂ mixtures. The μc-Si:H thin film was formed on aSiN_(x) thin film treated with an Ar plasma. The image extends laterallyover an area of 2×2 μm².

FIG. 4 shows an atomic force microscopy relief of a μc-Si:H thin filmproduced from SiF₄-Ar-H₂ mixtures. The μc-Si:H thin film was formed on aSiN_(x) thin film treated with an N₂ plasma. The image extends laterallyover an area of 5×5 μm².

These drawings are provided for illustrative purposes only and shouldnot be used to unduly limit the scope of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention concerns a method for producing a transistor 1 for activematrix display comprising the steps of forming an active material 14 andelectrodes 2, said active material being formed using vapor depositionmethods and said transistor 1 comprising an insulator 3. The 20 vapordeposition methods use, for example, radiofrequency glow dischargetechnique. In a particular embodiment, one uses a 13.56 MHz PECVDreactor. However, said reactor can be powered by radiofrequency energyat another frequency. The vapor deposition methods can implement also amicrowave ECR (electron 25 cyclotron resonance) technique.

According to the invention, one forms a plasma treated interface 4 ontop of said insulator 3. In a preferred embodiment, said treatedinterface 4 is selected from the group consisting of a SiN_(x) layer, aSiN_(x)O_(y) layer, a SiO₂ layer and glass. The plasma treated interface4 can be formed using a gas selected from the group consisting of N₂,O₂, N₂O and NH₃.

One then forms on top of said treated interface 4, a microcrystallinefilm 5 at a temperature comprised between 100 and 400° C. using at leasta deposition chemical element and a crystallisation chemical element.Said crystallisation chemical element is said to promote thecrystallisation of the μc-Si:H thin film 5 and is, for example, H₂. Thecrystallisation chemical element and the deposition element can bedeposited together or alternately with time. The deposition chemicalelements are selected among the group comprising SiH₄, SiF₄. A buffergas can be added to optimise the plasma conditions, said gas beingchosen from the group consisting of Ar, Xe, Kr and He.

In a particular implementation, hydrogen is used through plasmadeposition as a crystallisation element for the formation of μc-Si:Hfilms. The hydrogen plasma exposure of a-Si:H films formed from puresilane crystallises said a-Si:H films through its surface and subsurfacereactions. The method then consists in repeating many times thedeposition of a-Si:H during a time τ₁ followed by its exposure to ahydrogen plasma during a time τ₂. The μc-Si:H thin film is said to beformed by plasma deposition from a SiH₄, H₂ mixture. In a preferredembodiment, the deposition chemical elements flux and thecrystallisation chemical elements flux are at equilibrium during thegrowth of the microcrystalline silicon film 5.

The microcrystalline silicon film 5 has a thickness which is comprisedbetween 100 nm and 450 nm and comprises grains 6 whose size rangesbetween 10 nm and 400 nm.

For the preparation of either bottom gate or top gate transistors, themicrocrystalline silicon film 5 is grown on an insulator 3. Highmobility microcrystalline silicon 5 is obtained when the insultor 3 istreated so that the density of nucleation sites is reduced. Thistreatment can be achieved either for bottom gate or for top gatetransistors.

In a first embodiment, a top gate transistor is formed, one patterns asubstrate comprising a metallic or a TCO (transparent conductive oxyde)layer to form source and drain electrodes 2.

In a second embodiment, one forms a bottom gate transistor and thesubstrate comprises a gate electrode 2. The invention also concerns atransistor 1 for active matrix display. FIG. 1 shows the said transistor1 according to a particular embodiment of the invention. It comprises amicrocrystalline silicon film 5 and an insulator 3, the crystallinefraction being above 80%. The transistor 1 for active matrix displayalso comprises a plasma treated interface 4 located between theinsulator 3 and the microcrystalline silicon film 5 so that the saidtransistor 1 has a linear mobility equal or superior to 1.5 cm²V⁻¹s⁻¹and shows threshold voltage stability. Preferentially the crystallinefraction is higher than 85% since an excellent voltage stability is thenachieved (the shift is less than 0.15 V).

According to the current understanding of said transistor 1 structure,the formation of a plasma treated interface 4 reduces the density ofnucleation sites, thus allowing the lateral growth of crystallites 6.Said growth leads to the formation of large grains 6 resulting in ameasured higher mobility.

Said microcrystalline silicon film 5 can be produced by hot wiretechnique or by radiofrequency glow discharge technique.

The microcrystalline silicon film 5 contains crystallites 6 that arecrystallised grains 6 having a size ranging between 10 nm and 400 nm.Advantageously, the grain size ranges between 100 nm and 200 nm.

In an embodiment, the microcrystalline silicon film 5 thickness iscomprised between 100 nm and 450 nm, and preferentially is comprisedbetween 100 nm and 150 nm to have a low OFF current. A low off current,besides a high linear mobility and stability, is required for anindustrial application since it determines the image quality in theelectro-optic transducer (liquid crystal display, organicelectroluminescent display, . . . ).

FIG. 1 shows a thin film transistor 1 for active matrix display having abottom-gate electrode, but the transistor 1 can also be realised with atop-gate electrode.

The invention further concerns a display unit having a line-columnmatrix of pixels comprising at least a transistor 1 as previouslydescribed. Said pixels are actively addressed which means that thematrix-type display is scanned line by line over the frame time and acurrent is supplied to said pixels during the whole frame time. Thisaddressing method makes it perfectly suitable to pixels comprising lightemissive organic or polymer materials. It can be advantageously used forpixels comprising liquid crystals. In a preferred embodiment, electroniccontrol means to drive each pixel are at least partially integrated onthe corresponding display panel. Hence, the number of external drivingcircuits is reduced.

The transistor for active matrix display and the method for producingthe said transistor according to the invention have been the object ofvarious implementations whose following examples demonstrate the qualityof the results obtained.

EXAMPLE 1

The method for producing a transistor 1 according to the invention hasbeen first implemented to study a bottom gate thin film transistorproduced from SiF₄-Ar-H₂ mixtures. FIG. 2 shows the experimental(circles, respectively inverted triangles) values obtained as a functionof the “percolation thickness” (nm) 7 for the linear mobility(cm²V⁻¹s⁻¹) 8 of said μc-Si:H thin film transistor. The “percolationthickness” is a parameter which is defined as the thickness at which thecrystalline fraction in the film reaches 100%. The dashed line 9(respectively solid line 10) Is only shown to provide a guide line tothe eye.

The circle values 11 were obtained for a μc-Si:H thin film formed byplasma deposition on a SiN_(x) substrate under a total pressure of 1Torr of SiF₄-Ar-H₂ mixtures and an RF power of 240 mW/cm²for 35 minutes.The inverted triangle values 12 were obtained for a μc-Si:H thin filmformed by plasma deposition at a RF power of 280 mW/cm² The pressure ofsaid mixture was 1.5 Torr. The square value 13 was obtained for aμc-Si:H thin film formed by submitting a SiNx substrate to a SiF₄-Ar-H₂mixture plasma treatment under a total pressure of 1 Torr and an RFpower of 280 mW/cm² for 60 minutes. All these depositions were performedat a temperature of 200° C.

From these experimental values, there is a clear tendency for bothseries of samples to an increase of the mobility with the increase ofthe percolation thickness. Hence, despite of having a slowcrystallisation velocity, films that end up being fully crystallisedhave the highest mobilities. Thin film transistors having a mobility of3 cm²/V.s. are reported.

EXAMPLE 2

FIG. 3 shows an atomic force microscopy (AFM) relief of a μc-Si:H thinfilm 5. The μc-Si:H thin film 5 was formed by submitting a SiN_(x) thinfilm 3 to an Ar plasma and then to a SiF₄-Ar-H₂ mixture plasmatreatment. The image extends laterally over an area of 5×5 μm².Measurements of the thin film transistor realised with said μc-Si:H thinfilm show values for the linear mobility of the order of 0.02 cm²/V.s.This AFM image clearly shows small crystallites 6 having a mean sizeless than 80 nm.

FIG. 4 shows an atomic force microscopy relief of a μc-Si:H thin filmproduced from a SiF₄-Ar-H₂ mixture. The μc-Si:H thin film was formed ona SiN_(x) thin film submitted to a N₂ treatment. The image extendslaterally over an area of 2×2 μm² Measurements of the thin filmtransistor realised with said μc-Si:H thin film show values for thelinear mobility of the order of 3 cm²/V.s. This AFM image clearly showssmall crystallites having a mean size of the order of 400 nm.

The formation of a plasma treated SiN_(x) interface 4 on top of theSiN_(x) layer 3 before the μc-Si:H thin film 5 deposition clearlypromotes the deposition of large grain 6 materials at low temperatures.The linear mobility of the film obtained is then shown to be superior to2 cm²/V.s.

The invention claimed is:
 1. A method for producing a transistor foractive matrix display comprising the steps of: forming an activematerial and electrode on a substrate, said active material being formedusing a vapor deposition method; forming an insulator on top of saidactive material and electrode; forming a plasma treated interface on topof said insulator; and forming a microcrystalline silicone film on topof said treated interface at a temperature between 100 and 400° C. usingat least a deposition chemical element and a crystallisation chemicalelement, wherein, said microcrystalline silicon film comprises acrystalline fraction of above 80% and said microcrystalline silicon filmcomprises grains of a size between 10 nm and 400 nm, said plasma treatedinterface is selected from the group consisting of a SiN_(x) layer, aSiN_(x)O_(y) layer, a SiO₂ layer and glass, said plasma treatedinterface is formed using a gas, said gas selected from the groupconsisting of N₂, 0₂, N₂0 and NH₃, and the transistor has a linearmobility greater than or equal to 1.5 cm²V⁻¹ s⁻¹.
 2. The method forproducing a transistor according to claim 1, wherein themicrocrystalline silicon film is formed using a buffer gas selected fromthe group consisting of Ar, Xe, Kr and He.
 3. The method for producing atransistor according to claim 1, wherein said crystallisation chemicalelement is H₂.
 4. The method for producing a transistor according toclaim 1, wherein said deposition chemical element is selected from thegroup consisting of SiH₄ and SiF₄.
 5. The method for producing atransistor according to claim 1, wherein said deposition chemicalelement generates a flux and said crystallisation chemical elementgenerates a flux, both of which are at equilibrium during the growth ofthe microcrystalline silicon film.
 6. A method for producing atransistor according to claim 1, wherein a bottom gate transistor isformed.
 7. The method for producing a transistor according to claim 1,wherein the microcrystalline silicon film thickness is comprised between100 nm and 450 nm.
 8. The method for producing a transistor according toclaim 1, wherein the microcrystalline silicon film is produced by a hotwire technique.
 9. The method for producing a transistor according toclaim 1, wherein the microscrystalline silicon film is produced by aradiofrequency glow discharge technique.
 10. The method for producing atransistor according to claim 1, wherein the vapor deposition methodsuse a radiofrequency glow discharge technique.
 11. The method forproducing a transistor according to claim 10, wherein the vapordeposition methods uses a 13.56 MHz PECVD reactor.